The present application is the national stage under 35 U.S.C. 371 of PCT/IT99/00211, filed Jul. 9, 1999.
The invention relates to a drawing process where the metal wire is coated with a thin surface layer of material to improve drawing, the wire being passed through several different drawing holes.
A process of the above-described type is known, in which the wire is coated with a lubricant which serves both to reduce friction during the forced passage through the draw-plate and to obtain a wire with a smooth surface and a constant section. Prior-art drawplates exhibit a reduction tract, where the metal wire is subjected to plastic deformation. This reduction tract has an angle of convergence which is normally comprised between 12 and 14xc2x0, but in any case never exceeds 20-25xc2x0. EP-A-0 537 618 discloses a process for metal wire drawing in which a metal wire, previously coated by a layer of material for aiding the subsequent drawing operation, is drawn by means of a series of drawing operations. The wire passes through a series of draw-plates in order to reach its final section size. The draw-plates have holes each exhibiting a reduction tract which produce a plastic deformation of the wire. The angle of convergence of the reduction tracts is less than 25xc2x0.
This is due to the fact that drawplates wit holes having an angle of convergence above this produce irregular drawing results thanks to excessive traction forces on the wire, which can be exacerbated to the point of breakage in further drawing operations. In any case, such levels of traction lead to rapid wear on the wire. Prior-art drawing processes of the above type generally comprise a further phase, after the last of the drawing operations, in which the wire is treated to remove the surface layer of lubricant. This leads to several drawbacks. Firstly, the process is made complicated by the need to predispose suitable equipment for removing the lubricant from the wire after the process; secondly, working times are considerably affected, with consequent increases in manufacturing costs.
The main aim of the present invention is to eliminate the above-described drawbacks by providing an improved drawing process in which a surface layer of the wire can be removed simply and economically.
An advantage of the invention is that after the final drawing stage a subsequent phase is not necessarily required in order to remove a coating from the wire. A further advantage of the present invention is that a high-quality metal wire is obtained, having a smooth external surface and straight constant transversal section.
A further aim of the present invention is to provide a tool which simply and economically removes a surface layer of coating from the metal wire being drawn.